This invention relates to an improved method of edging a contact lens or lens blank. Additionally, the invention provides a method for producing a contact lens having a desired edge profile.
Various processes are known for curing a monomeric mixture in the production of contact lenses, including spincasting and static casting. Spincasting methods are disclosed in U.S. Pat. Nos. 3,408,429 (Wichterle) and 3,660,545 (Wichterle), and static casting methods are disclosed in U.S. Pat. Nos. 4,113,224 (Clark et al.) and 4,197,266 (Clark et al.). In spincasting processes, the monomeric mixture is charged to a mold member having a concave molding cavity for forming the anterior lens surface, and the posterior lens surface is formed from rotation of the mold. In static casting processes, a monomeric mixture is cured between a first mold member including a concave molding cavity for forming the anterior lens surface and a second mold member including a convex molding cavity for forming the posterior lens surface. An additional method, disclosed in U.S. Pat. No. 4,555,732 (Kunzler et al.), involves curing an excess of a monomeric mixture in a concave anterior mold member by spincasting to form a lens blank having an anterior lens surface and a relatively large thickness, followed by lathe cutting the posterior surface of the lens blank to obtain a contact lens having a desired thickness and posterior lens surface.
Often, it is desirable, or necessary, to machine the edge of a cast contact lens in order to provide a contact lens more suitable for placement on the eye. For example, it may be desirable to modify the edge profile so that the resultant contact lens has an edge which is less irritating to the eye, or it may be necessary to remove excess flash at the edge of the cast article.
FIG. 1 schematically illustrates a contact lens 1 which has been cast in mold member 2, such as by a conventional spincasting process. Mold member 2 includes: a support, which for this mold embodiment is cylindrical shell 3; concave molding cavity 4 having a central axis 5; and right cylinder wall 6 (i.e., a cylindrical wall at the periphery of the molding cavity which is normal to planar surface 7). As seen more clearly in FIG. 2, a sectional enlargement of FIG. 1 (noting, however, that the figures are not drawn to scale in order to illustrate better the edge profiles, e.g.), the spincast lens 1 has edge 8. Usually, however, this edge will actually have a more irregular shape; for example, edge 8 may include flash formed from the casting operation. In such a case, the lens would be even less suitable for placement on the eye.
Conventional methods for edging contact lenses include trimming the lens edge by cutting away a peripheral portion of the lens edge along with an adjacent section of the mold member while the lens remains secured in the anterior mold member. Such methods are disclosed in U.S. Pat. Nos. 3,832,920 (Wrue), 3,896,688 (Wrue), 4,534,723 (Dillon et al.) and 4,584,148 (Rawlings et al.).
Such conventional methods are illustrated representatively in FIG. 3, wherein the peripheral sections of the lens edge and the mold member which were removed from the assembly of FIG. 2 are indicated by dashed line 9. More specifically, the lens edge is circumferentially cut with a knife edge along line 10, wherein the knife edge is positioned at 90.degree. with respect to the central axis of the mold member. Such a procedure is further illustrated in FIG. 4 of U.S. Pat. Nos. 3,832,920 and 3,896,688, and FIGS. 1 and 7 of U.S. Pat. Nos. 4,534,723 and 4,584,148.
As a refinement of these methods, it is known to round the posterior edge juncture 12 (FIG. 3) formed from this knife cutting, such as by mechanical buffing or polishing, in order to reduce the sharpness at this juncture. Accordingly, the lens edge assumes a shape as representatively shown in FIG. 4.
Nonetheless, it has been found that the conventional methods do not always provide a contact lens which is suitable for placement on the eye. For example, contact lenses produced by the conventional methods may cause irritation to the eye such as unacceptable levels of conjunctival staining.